In a transmissive liquid crystal display device, in order to display an image, a backlight device that applies light from the back surface of a display unit (liquid crystal panel) is required. Conventionally, cold cathode tubes called CCFLs have been widely adopted as a light source of the backlight device. However, in recent years, light emitting diodes (LEDs) have been increasingly adopted from the viewpoint of low power consumption and the ease of luminance control.
Focusing on the structure, the backlight device is mainly classified into a so-called “direct type” in which the light source is disposed on the back surface of the display unit and a so-called “edge-light type” in which the light source is disposed at the side edge portion of the display unit. According to the direct type backlight device using an LED as the light source, it is possible to increase the luminance by arranging a large number of LEDs on the back surface of the display unit and to perform a finer local dimming process (described later). On the other hand, according to the edge-light type backlight device using the LED as the light source, it is possible to reduce the thickness and to reduce the power consumption due to the smaller number of LEDs used compared to the direct type.
In recent years, in order to reduce the power consumption, there has been developed a liquid crystal display device that performs the local dimming process in which a screen is logically divided into a plurality of areas and the luminance (light emission intensity) of the light source (typically, LED) is controlled for each area. In the local dimming process, the luminance of the light source is controlled based on an input image in the corresponding area. Specifically, the luminance of each light source is obtained based on the maximum value, an average value, or the like, of target luminance (luminance corresponding to the input tone value) of pixels included in the corresponding area. In an area where the luminance of the light source is made smaller than the original luminance, the transmittance of each pixel is increased. As a result, targeted display luminance can be obtained in each pixel. Further, in recent years, HDR drive for displaying an extremely wide dynamic range has been developed actively. The local dimming process is used also at the time of achieving this HDR driving.
Three conventional examples concerning the circuit configuration of the edge-light type backlight device (the configuration of the backlight drive circuit) will be described. In the following first to third conventional examples, it is assumed that four LEDs 91(1) to 91(4) are provided as light sources on one end side (e.g., upper end side) of the display unit. In any of the following first to third conventional examples, as shown in FIG. 31, four LEDs 91(1) to 91(4) are arranged side by side on an LED mounting board 910 provided on one end side of the display unit. In the following description, when it is unnecessary to distinguish the four LEDs 91(1) to 91(4) from each other, the LED is simply denoted by numeral 91. The same applies to other constituent elements.
FIG. 32 is a circuit diagram showing the configuration of the backlight drive circuit in the first conventional example. Note that the configuration of the first conventional example is a configuration that can be adopted in a display device in which the local dimming process is not performed. The backlight drive circuit shown in FIG. 32 is constituted by four LEDs 91(1) to 91(4), a DC-DC converter 901, a DC-DC control unit 902, a FET 905, and a sense resistor 906. The four LEDs 91(1) to 91(4) are connected in series. An anode terminal of: the LED 91(1) disposed most upstream is connected to an output terminal of the DC-DC converter 901, and a cathode terminal of the LED 91(4) disposed most downstream is connected to a drain terminal of the FET 905.
Based on the control by the DC-DC control unit 902, the DC-DC converter 901 converts an input direct-current voltage (input voltage DCDC_VIN) into a direct-current voltage (output voltage DCDC_VOUT) necessary for causing all of the LEDs 91(1) to 91(4) to emit light with desired luminance. Based on a voltage obtained by the sense resistor 906, the DC-DC control unit. 902 controls on/off of switches (e.g., switch on the high side and a switch on the low side) in the DC-DC converter 901 so that the output voltage DCDC_VOUT has a desired magnitude. In addition, the DC-DC control unit 902 controls a gate voltage of the FET 905 based on the voltage obtained by the sense resistor 906 so that a constant current is supplied to the LEDs 91(1) to 91(4). As thus described, the constant current drive circuit is achieved by the FET 905, the sense resistor 906, and a part of the DC-DC control unit 902.
With the configuration and operation as described above, even when there occurs a change in a forward voltage due to a change in temperature or the like in each of the LEDs 91(1) to 91(4), a constant current is supplied to the LEDs 91(1) to 91(4), and hence the LEDs 91(1) to 91(4) emit light with constant luminance.
According to the first conventional example, all of the LEDs 91(1) to 91(4) emit light with same luminance. Therefore, the configuration of the first conventional example cannot be adopted in the display device on which the local dimming process is performed.
FIG. 33 is a circuit diagram showing the configuration of the backlight drive circuit in the second conventional example. Note that the configuration of the second conventional example is a configuration that can be adopted in a display device on which the local dimming process is performed. The backlight drive circuit shown in FIG. 33 is constituted by four LEDs 91(1) to 91(4) corresponding to four areas, respectively, four FETs 915(1) to 915(4) corresponding to the four LEDs 91(1) to 91(4), respectively, four sense resistors 916(1) to 916(4) corresponding to the four LEDs 91(1) to LED 91(4), respectively, a DC-DC converter 901, a DC-DC control unit 902, a switch control unit 904, an FET 905, and a sense resistor 906. In the second conventional example, unlike the first conventional example, the four LEDs 91(1) to 91(4) are connected in parallel. Then, the switch control unit 904 controls the gate voltages of the four FETs 915(1) to 915(4) based on the voltages obtained by the four sense resistors 916(1) to 916(4), respectively, so that the four LEDs 91(1) to 91(4) emit light with desired luminance.
As understood from FIG. 33, according to the second conventional example, a constant current drive circuit constituted by the FET 915, the sense resistor 916, and a part of the switch control unit 904 is required in number equal to the number of areas. In addition, there is a concern that a voltage loss at the FET 915 and the sense resistor 916 increases and the driving efficiency of the LED deteriorates. Therefore, the configuration of the third conventional example described below has been proposed.
FIG. 34 is a circuit diagram showing the configuration of the backlight drive circuit in the third conventional example. The backlight drive circuit shown in FIG. 34 is constituted by four LEDs 91(1) to 91(4) corresponding to four areas, respectively, four switches 903(1) to 903(4) corresponding to the four LEDs 91(1) to 91(4), respectively, a DC-DC converter 901, a DC-DC control unit 902, a switch control unit 904, an FET 905, and a sense resistor 906. As understood from FIG. 34, the four LEDs 91(1) to 91(4) are connected in series. Further, each switch 903 is connected in parallel with the corresponding LED 91.
In the above configuration on, the switch control unit 14 controls on/off of the four switches 903(1) to 903(4). Thus, the supply of the current to each LED 91 is controlled, and the luminance is adjusted for each LED 91.
According to the third conventional example, unlike the second conventional example, the number of constant current drive circuits is not necessarily equal to the number of areas. Thus, at the time of performing the local dimming process, favorable driving efficiency can be obtained as compared to the second conventional example.